This invention is directed to a cathodic electrocoating composition and in particular to a cathodic electrocoating composition containing primary metal catalyst for crosslinking of the compsition on curing.
The coating of electrically conductive substrates by an electrodeposition process (also called an electrocoating process) is a well known and important industrial process. Electrodeposition of primers to automotive substrastes is widely used in the automotive industry. In this process, a conductive article, such as an autobody or an auto part, is immersed in a bath of a coating compostion of an aqueous emulsion of film forming polymer and acts as an electrode in the electrodeposition process. An electric current is passed between the article and a counter-electrode in electrical contact with the aqueous emulsion, until a desired coating is deposited on the article. In a cathodic electrocoating process, the article to be coated is the cathode and the counter-electrode is the anode.
Resin compositions used in the bath of a typical cathodic electrodeposition process also are well known in the art. These resins typically are made from polyepoxide resins which have been chain extended and then an adduct is formed to include amine groups in the resin. Amine groups typically are introduced through reaction of the resin with an amine compound. These resins are blended with a crosslinking agent and then neutralized with an acid to form a water emulsion which is usually referred to as a principal emulsion.
The principal emulsion is combined with a pigment paste, coalescent solvents, water, and other additives to form the electrocoating bath. The electrocoating bath is placed in an insulated tank containing the anode. The article to be coated is the cathode and is passed through the tank containing the electrodeposition bath. The thickness of the coating deposited on the article is a function of the bath characteristics, the electrical operating characteristics, the immersion time, and the like.
The coated article is removed from the bath after a certain period of time. The article is rinsed with deionized water and the coating is cured typically in an oven at sufficient temperature to produce a crosslinked coating.
Cathodic electrocoating compositions and resin compositions, coating baths, and cathodic electrodeposition processes are disclosed in U.S. Pat. Nos. 3,922,253; 4,419,467; 4,137,140; and 4,468,307.
Cathodic electrocoating compositions contain catalysts which increase the curing rate and reduce curing temperatures of the composition after it has been applied. Examples of known catalysts that are used in electrocoating composition are dibutyl tin oxide and dibutyl tin dilaurate. Thses catalysts are used in amounts of about 0.25-2% by weight, based on the total weight resin, in the composition. However, dibutyl tin oxide is a solid which is difficult to disperse and keep dispersed in an electrocoating bath. Dibutyl tin dilaurate can be hydrolyzed in an electrocoating bath and form lauric acid which causes film appearance problems such as cratering. A catalyst is needed that is readily dispersed and remains dispersed in the electrocoating bath.